A typical passenger conveyor, such as an escalator or moving walk, includes a truss, a plurality of sequentially connected treadplates traveling through a closed loop path within the truss, and a machine for driving the treadplates. A control system is used to control the operation of the conveyor for such parameters as speed and direction. Such conveyors are a very efficient means to move large numbers of passengers between floors or locations.
In modem conveyors, the control systems have become increasingly complex in an effort to improve efficiency and reduce wear and cost of operation. One such control system incorporates a sensor to detect passengers entering the conveyor. Upon detection of a passenger, the conveyor is switched on and the treadplates begin moving. After the passenger exits the conveyor, the conveyor is switched off. This type of control system saves energy by not running continuously during no-load conditions. A drawback to this system, however, is the generation of peak loads during the frequent switching on and off of the conveyor.
One disclosed solution to this problem is to use a variable frequency converter to control the operation of the conveyor. An example of this type of controller is disclosed in U.S. Pat. No. 4,748,394, entitled "Control Apparatus for Escalator" and issued to Watanabe. This patent discloses an escalator operating at two speeds. At low speed, the motor is controlled by the frequency conversion means to operate at the low speed until a passenger is detected. Upon detection, the frequency is gradually raised until there is synchronization between the current generated from the frequency conversion means and the power source. At that point, the motor is switched to the power source. A similar concept is proposed in published German Patent Application No. 4,313,424 A1. While this solution may provide some benefits, even when operated at a reduced speed wear of the escalator components still occurs and energy is wasted during no-load conditions.
The above art notwithstanding, scientists and engineers under the direction of Applicant's Assignee are working to develop drives and control systems for passenger conveyors that improve efficiency and minimize wear.